1. Field of the Invention
The present invention relates to a computer system or a memory unit used for peripheral equipments of the computer, and more particularly to a memory unit detectable of the existence of gain margin of a servo system at a head positioning mechanism for a head of the memory unit.
2. Description of the Prior Art
FIG. 1 is a schematic side view of mechanical structure of a magnetic disc apparatus as an example of the conventional memory unit, which has been disclosed in, for example, the Japanese Patent Laid-Open Gazette No. 60-101777 (1985).
In FIG. 1, reference numeral 1 designates a support base for the entire apparatus, 2 designates a rotor comprising a spindle motor 2M installed on the support base 1 and a rotary shaft 2a for the motor 2M, and 3 designates a disc like recording medium mounted on the rotary shaft 2a, which herein uses a magnetic disc.
Reference numeral 4 designates a head disposed opposite to a recording surface of the recording medium 3, 5 designates a pressure spring for biasing the head 4 toward the recording surface of the recording medium 3, and 6 designates an arm supporting one end of the pressure spring 5.
Reference numeral 7 designates an actuator for moving the head 4 radially of the recording medium 3 so as to position the head 4, the actuator comprising a carriage 8 fixedly supporting one end of the arm 6, a bearing 9 which moves and guides the carriage 8 on the support base 1, a cylindrical coil 10 provided at one end of the carriage 8, a magnet 11 disposed opposite to the coil 10, and a yoke 12 for positioning the magnet 11.
Reference numeral 13 designates a sensor for detecting the position of head 4, which comprises a converter 14 fixed on the support base 1 and an optical linear scale 15 disposed at the carriage 8 side.
A drive control unit at such memory unit has generally been constructed as shown in FIG. 2.
In FIG. 2, reference numeral 16 designates a velocity demodulator which differentially shapes the position information of the head 4 given from the sensor 13 so as to detect the positioning speed of head 4, and 17 designates a servo circuit for driving the actuator 7 while comparing the position information from the sensor 13 given a positioning instruction from a computer 19 with the velocity information from the velocity demodulator 16.
Reference numeral 18 designates a read/write circuit which reads and reproduces the information from the recording medium 3 through the head 4 and writes and stores that in the recording medium 3.
Reference numeral 19 designates a computer which outputs the moving and positioning instruction for the head 4 to the servo circuit 17, or outputs a recording and reproducing instruction to the read/write circuit 18.
Such conventional memory unit operates as follows:
When the moving and positioning instruction is given by the computer 19 to the servo circuit 17, the servo circuit 17 gives to the actuator 7 a positioning signal for the head 4 while fetching from the sensor 13 the present position information of head 4 and from the velocity demodulator 16 the positioning and moving speed of the head 4, thereby moving the head 4 toward a predetermined position and positioning it thereat.
When the head 4 is positioned at the predetermined position, a recording or reproducing instruction is delivered from the computer 19 to the read/write circuit 18 to allow the head 4 to perform recording on the recording medium 3 and the recording medium 3 to reproduce the record, so that the computer 19 is adapted to read out it through the read/write circuit 18.
In the aforesaid conventional memory, the head 4 is positioned with respect to the recording medium 3 by a servo mechanism comprising the servo circuit 17, actuator 7, sensor 13 and velocity demodulator 16, and on the basis of the usual automatic control theory, and the gain margin is required to be over a predetermined value, usually several dB, for stable operation of the servo mechanism.
The actuator 7 and sensor 13 or the like, however, are composed of combination of a plurality of parts respectively, whereby the problem is created in that mechanical resonance is easy to occur due to variation in the parts and assembly thereof, the gain margin is small, the servo mechanism is unstable to cause vibrations, and the head 4 cannot be positioned with accuracy.